It is known in the art that the presence of water is often deleterious when melt forming thermoplastic resins. If sufficient water is present, the steam formed when the resin is heated above 100.degree. C. may cause the resulting part to contain undesirable foam or bubbles. More insidiously, many commercially important resins, such as nylons and polyesters, are chemically degraded by hydrolysis with water, so that the physical properties of the resulting parts are deficient. This is not obvious from simple visual inspection of the part; it usually is detected by destructive testing of the part, or by premature failure of the part in use.
Equipment for drying and conveying resins is expensive to install and maintain, and the necessity for drying resin results in increased costs. It is found in practice that unnecessary costs are sustained when resin is routinely dried, whether or not it actually needs to be, because the moisture level is not known. At times, the drying equipment itself will undetectably malfunction, possibly resulting in humidified rather than dried resin. Certain analytical methods used to test for moisture content depend upon an assumption of homogeneity unlikely to be correct in a large batch of resin which has been exposed to moist air. On-line methods of moisture analysis are very costly.
U.S. Pat. No. 5,078,909 of Shigeta et al. issued Jan. 7, 1992, discloses a moisture-absorbent composition containing a thermoplastic resin and a moisture indicator agent. The compositions have an indicator function having a color variable in accordance with the rate of moisture absorption and can be used to form molded items. No mention is made of control of timing of the color change to correlate with a specific moisture level.
U.S. Pat. No. 4,018,061 issued Apr. 19, 1977, of Williamitis discloses a moisture indicator for refrigerant systems in which a moisture indicating compound, such as a cobalt salt, is mixed with a hygroscopic plastic to indicate when the refrigerant is wet. The mixture is described as being applied from solution as a paint. No mention is made of melt forming mixtures or blends of the plastic and moisture indicator.
U.S. Pat. No. 4,743,238 issued May 10, 1988, and U.S. Pat. No. 4,681,576 issued Jul. 21, 1987, each of Colon et al., describe a hot-melt adhesive containing a moisture indicator, which causes the adhesive to be different colors in the wet and dry states. No mention is made of melt forming shaped parts from the adhesive, which is a complex mixture of polymers, fatty acids, and other adjuvants.
U.S. Pat. No. 4,263,850 issued Apr. 28, 1981, of Bouchard et al., discloses a pyrotechnic cap containing a moisture indicator composition in powdered form of cobalti, cobaltous cyanide and a plastic such as a polyolefin. No mention is made of melt forming shaped parts from the composition, or that the plastic used is hydrolytically unstable.
Japanese Patent 89/201,364, published Aug. 14, 1989, describes a moisture indicating "packet" in which CoCl.sub.2 is absorbed by a crosslinked polymer and sealed inside a water permeable plastic bag. By definition, crosslinked resins cannot be melt formed.
Japanese Patent 82/127093, published Jul. 21, 1982, describes a moisture indicating film made from a complex chemical mixture, but eventually involving the reaction of iodate ion with iodide ion in the presence of water to form iodine. The iodine then interacts with starch present to form the colored starch-iodide complex. No mention is made of using this as a molding resin.
U.S. Pat. No. 4,959,445 of Rosenfeld issued Sep. 25, 1990, discloses the polymerization of certain monomers to make aromatic polyesters (polyarylates) in the presence of cobalt salts. The resulting polymers have gray, blue or purple tints, as opposed to polymer which is made without cobalt compounds, which is usually yellow. No mention is made in this patent that these colors are variable or change with the moisture content of the aromatic polyester. No mention is made of adding cobalt compounds to the polyester after the polymerization is completed.
The present invention provides thermoplastic molding resins which exhibit a color change, preferably at or below their critical water concentration, easily determined by simple visual inspection, thereby greatly decreasing the undesirable possibility that wet resin will be melt formed.
The present invention further provides moisture indicator compositions containing a moisture indicator and one or more thermoplastic resins for use in melt forming thermoplastic resins.
The present invention provides methods or processes for melt forming shaped parts or articles from thermoplastic resins containing a moisture indicator.
The present invention further provides a method for monitoring the moisture content of thermoplastic resins for proper melt forming.
The present invention further provides melt formed articles made from thermoplastic resins containing a moisture indicator such that the articles exhibit a color difference between their wet and dry states.